One of the most photogenic caves on earth! :D It is located within very active geothermal area and since the 18th century was a popular place for natural bathing. However, since eruptions at the end of the last century, the water heated up to the temperature beyond safe. Nevertheless, it is incredibly charming with its magnificent blue water, all the fumes above the surface and the rays of light entering through the holes in the ceiling and narrow entrances.
Like most of "tourist attractions" in Iceland, it is located on a private land. If you will ever visit it (and I strongly suggest you do!), please have that in mind, respect the will and all requests of the owner, and leave no trace. BTW - ceiling could collapse any minute. Just saying. ;)
I'll gladly hear your thoughts!